Introduce request attributes in RestClient

This commit introduces request attributes in the RestClient and
underlying infrastructure (i.e. HttpRequest).

Closes gh-32027

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilder.java

@@ -18,6 +18,8 @@ package org.springframework.web.reactive.function.client;
 
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
+import java.util.Collections;
+import java.util.Map;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
@@ -65,6 +67,11 @@ final class DefaultClientResponseBuilder implements ClientResponse.Builder {
 		public HttpHeaders getHeaders() {
 			return HttpHeaders.EMPTY;
 		}
+
+		@Override
+		public Map<String, Object> getAttributes() {
+			return Collections.emptyMap();
+		}
 	};
 
 

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/ExchangeFunctions.java

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.function.client;
 
 import java.net.URI;
+import java.util.Map;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -149,6 +150,11 @@ public abstract class ExchangeFunctions {
 					return request.url();
 				}
 
+				@Override
+				public Map<String, Object> getAttributes() {
+					return request.attributes();
+				}
+
 				@Override
 				public HttpHeaders getHeaders() {
 					return request.headers();

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultServerRequestBuilder.java

@@ -193,7 +193,7 @@ class DefaultServerRequestBuilder implements ServerRequest.Builder {
 	@Override
 	public ServerRequest build() {
 		ServerHttpRequest serverHttpRequest = new BuiltServerHttpRequest(this.exchange.getRequest().getId(),
-				this.method, this.uri, this.contextPath, this.headers, this.cookies, this.body);
+				this.method, this.uri, this.contextPath, this.headers, this.cookies, this.body, this.attributes);
 		ServerWebExchange exchange = new DelegatingServerWebExchange(
 				serverHttpRequest, this.attributes, this.exchange, this.messageReaders);
 		return new DefaultServerRequest(exchange, this.messageReaders);
@@ -220,8 +220,10 @@ class DefaultServerRequestBuilder implements ServerRequest.Builder {
 
 		private final Flux<DataBuffer> body;
 
+		private final Map<String, Object> attributes;
+
 		public BuiltServerHttpRequest(String id, HttpMethod method, URI uri, @Nullable String contextPath,
-				HttpHeaders headers, MultiValueMap<String, HttpCookie> cookies, Flux<DataBuffer> body) {
+				HttpHeaders headers, MultiValueMap<String, HttpCookie> cookies, Flux<DataBuffer> body, Map<String, Object> attributes) {
 
 			this.id = id;
 			this.method = method;
@@ -231,6 +233,7 @@ class DefaultServerRequestBuilder implements ServerRequest.Builder {
 			this.cookies = unmodifiableCopy(cookies);
 			this.queryParams = parseQueryParams(uri);
 			this.body = body;
+			this.attributes = attributes;
 		}
 
 		private static <K, V> MultiValueMap<K, V> unmodifiableCopy(MultiValueMap<K, V> original) {
@@ -273,6 +276,11 @@ class DefaultServerRequestBuilder implements ServerRequest.Builder {
 			return this.uri;
 		}
 
+		@Override
+		public Map<String, Object> getAttributes() {
+			return this.attributes;
+		}
+
 		@Override
 		public RequestPath getPath() {
 			return this.path;

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientResponseTests.java

@@ -19,6 +19,7 @@ package org.springframework.web.reactive.function.client;
 import java.net.InetSocketAddress;
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.OptionalLong;
@@ -324,6 +325,11 @@ class DefaultClientResponseTests {
 			public HttpHeaders getHeaders() {
 				return HttpHeaders.EMPTY;
 			}
+
+			@Override
+			public Map<String, Object> getAttributes() {
+				return Collections.emptyMap();
+			}
 		};
 
 		given(mockExchangeStrategies.messageReaders()).willReturn(